There's lots of things that come to mind and not all of them are transmission related. Many exhaust parts can rattle or buzz as the drive live changes position under load so look at all of the mounts and brackets, which at 21 years old can be dry or broken. Heat shields on the pipes can cause similar sounds as well. Do not remove them but strap them down to the pipe with a large hose clamp to get a few more years of service.

That said, a cracked torque plate (flywheel for an automatic) can make serious noise too. That usually sounds more like a rod bearing but can vary between different cars. Sometimes they buzz and grind before failing completely.